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A Level Chemistry > Module 6 (chapter 25) - aromatic compounds > Flashcards

Module 6 (chapter 25) - aromatic compounds Flashcards

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1
Q

background of benzene

A
  • colourless, sweet smelling, flammable liquid
  • found naturally in crude oil, a component of petrol and found in cigarette smoke
  • classified as a carcinogen
  • hexagonal rings of six carbon atoms with each carbon joined to one hydrogen atom
  • classed as an arene
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2
Q

derivatives of benzene

A
  • benzaldehyde has the flavour of almonds

- thymol is found in the aromatic herb, thyme

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3
Q

Kekule’s model

A
  • 6 carbon joined by alternating single and double carbon bonds
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4
Q

how does the lack of reactivity disprove Kekule’s modern

A
  • if benzene contained a C=C bond it should declourise bromine in an electrophilic addition reaction
  • benzene does not decolourise bromine under normal conditions
  • this led scientists to suggest that benzene cannot have any C=C bonds in its structure
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5
Q

how does the length of carbon-carbon bonds disprove Kekule’s theory

A
  • X-ray diffraction used to measure bond lengths
  • in benzene all bounds were found to be the same length (0.139nm)
  • this was between the length of the single bond (0.153nm and a double bond of 0.134nm)
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6
Q

how does hydrogenation enthalpies disprove Kekule’s theory

A
  • if benzene did have the Kekule’s structure, then it would be expected to have an enthalpy change of hydrogenation that is three time that of cyclohexane
  • when cyclohexane is hydrogenated, one double bond reacts with hydrogen
  • the enthalpy change of hydrogenation is -120KJmol-1 an d so expected enthalpy is 360Kjmol-1
  • the actual enthalpy change is hydrogenation of benzene is only 208kjmol-1
  • this shows that the actual structure of benzene is more stable than the theoretical kekule’s model of benzene
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7
Q

the delocalised model of Benzene

A
  • benzene is planar
  • each carbon atom uses three of its available four electrons in bonding to two other carbon atoms and one to a hydrogen atom
  • each carbon atom has one electron in a a p-orbital at right angles to the plane of the bonded carbon and hydrogen atoms
  • adjacent p-orbitals electrons overlap sideways, in both directions, above and below the plane of the carbon atoms to form a ring of electron density
  • this overlapping of the p-orbitals creates a system of pi-bonds which spread over all six of the carbon atoms in the ring structure
  • the six electrons occupying this system of pi-bonds are said to be delocalised
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8
Q

compounds with one substituent group

A
  • in aromatic compounds, the benzene ring is often considered to be the parent chain
  • alkyl groups, halogens and nitro groups are all considered the prefixes to benzene
  • when a benzene ring is attached to an alkyl chain with a functional group or an alkyl chain with seven or more carbon atoms, benzene is considered a substituent
  • this means the prefix phenyl is used
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9
Q

exceptions to substituent rule

A
  • benzoic acid
  • phenylamine
  • benzaldeyde
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10
Q

compounds with more than one substituent group

A
  • this means the ring is now numbered

- substituent groups are listed in alphabetical order using the smallest numbers possible

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11
Q

benzene reactions

A
  • most are electrophilic substitution

- hydrogen atom is replaced by another atom or group of atoms

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12
Q

nitration of benzene

A
  • benzene reacts slowly with nitric acid to form nitrobenzene
  • this reaction is catalysed by sulphuric acid and heated at 50 degrees
  • water bath used to maintain a steady temperature
  • one of the hydrogen atoms is replaced by a nitro group (NO2)
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13
Q

why must nitration happen at 50 degrees

A

-if it goes above 50 degrees, further substitution reactions may occur leading to the production of dinitrobenzene

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14
Q

uses of nitrobenzene

A
  • preparation of dyes, pharmaceuticals and pesticides

- e.g. starting material to make paracetamol

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15
Q

electrophile in nitration of benzene

A

-nitric acid isn’t the electrophile

STEP 1:it is the nitronium ion NO2+ produced by the reaction of concentrated nitric acid with concentrated sulphuric acid

STEP 2: the electrophile accepts a pair of electrons from the benzene ring to forma a dative covalent bond
-the organic intermediate formed is unstable and breaks down to form the organic product nitrobenzene and a H+ion

STEP 3:H+ ion reacts with HSO4- ion from step 1 to regenerate the catalyse

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16
Q

halogenation of benzene

A
  • the halogens do not react with benzene unless a catalyst called a halogen carrier is present
  • common carriers are AlCl3, FeCl3, AlBr3, FeBr3
  • can be generated in situ from the metal and halogen
17
Q

bromination of benzene

A

-room temperature and in presence of a halogen carrier
-one of the hydrogen atoms is replaced by a bromine atom
STEP 1: benzene is too stable to react with a non-polar bromine molecule. the electrophile is the bromonium ion, Br+ which is generated when the electron carrier catalyst reacts with bromine

STEP 2: the. bromonium ion accepts a pair of electrons from the benzene ring to form a dative covalent bond. the organic intermediate is unstable and breaks down to form the organic product bromobenzene and an H+ ion

STEP 3: the H+ ion formed in step 2 reacts with the FeBr4- ion to regenerate the FeBr3 catalyst

18
Q

chlorination of benzene

A

-reacts in the same way as bromine but the halogen carrier used is AlCl3 of FeCl3…

19
Q

alkylation of benzene

A
  • the substitution of a hydrogen atom in the benzene ring by an alkyl group.
  • reaction is carried out by reacting benzene with a haloalkane in the presence of AlCl3 which acts as a halogen carrier catalyst, generating the electrophile
  • sometime called Friedal crafts alkylation
20
Q

acylation reactions

A
  • when benzene reacts with an acyl chloride in the presence of an AlCl3 catalyst, an aromatic ketone is formed
  • for example when ethanoyl chloride reacts with benzene it forms penylethanone (used in perfume)
21
Q

does cyclohexane decolourise bromine

A

-yes bromine adds across the double bond
-the pi-bond in the alkene contains localised electrons above and below the plane of the two bonded carbon atoms
this produces an area go high electron density
-the localised electrons in the pi-bond induce a dipole in the non-polar bromine molecule making one bromine atom slightly negative
-the slightly positive bromine atom enables the bromine molecule the act like an electrophile

22
Q

benzene doesn’t react like cyclohexane

A
  • doesn’t react with bromine unless a halogen catalyse is present
  • this is because benzene has a delocalised pi-electron spread above and below the plane of the carbon atoms in the ring structure
  • the electron density around any two carbon atoms in the benzene ring is less than the C=C double bond in an alkene
23
Q

phenols

A 
class of aromatic compounds hydroxyl group directly bonded to the benzene ring
-an OH group bonded to a carbon side chain doesn't count
24
Q

phenol as a weak acid

A
  • less soluble in water than alcohols due to the presence of the no-polar benzene ring
  • large and complex benzene ring it is difficult to tesselate and form intermolecular bonds
  • seen by comparing the acid dissociation constant -when dissolved in water it partially dissociates forming the phenoxide ions and a proton
  • because of this ability it is classified as a weak acid.
25
Q

how does the acidity of phenol compare to others

A
  • more acidic than alcohols but less than carboxylic acids
  • ethanol does not react with sodium hydroxide or sodium carbonate
  • phenols and carboxylic acids react with solutions of strong bases such as aqueous sodium hydroxide
  • only carboxylic acids are strong enough acids to react with the weak base, sodium carbonate
  • the carboxylic acid will react with sodium carbonate (weak base) to produce carbon dioxide by phenol won’t
26
Q

properties of phenol

A
  • solid at room temp

- slightly soluble in water

27
Q

reaction of phenol and sodium hydroxide

A
  • reacts with sodium hydroxide to form the salt, sodium phenoxide and water in a neutralisation reaction
  • C6H5OH + NaOH — C6H5O-Na+ + H2O
28
Q

phenol with metal (e.g. sodium)

A

2Na + 2C6H5OH — 2C6H5O-Na+ + H2

29
Q

phenol and bromine water

A
  • reacts with aq bromine to form 2,4,6-tribromophenol (a white precipitate)
  • decolourises bromine and a halogen carrier is not needed
  • reaction occurs at room temperature
30
Q

nitration of phenol

A
  • reacts readily with dilute nitric acid at room temperature
  • a mixture of 2-nitrophenol and 4-nitrophenol is formed
31
Q

why is phenol more reactive than benzene

A
  • bromine and nitric acid react more readily with phenol
  • the increased reactivity is because of a lone pair of electrons from the oxygen p-orbital of the OH group being donated into the pi-system of phenol
  • this increases the electron density of phenol and it becomes a nucleophile
  • the increased electron density attract electrophiles more strongly than benzene
  • the aromatic ring is more susceptible to attack from electrophiles than in benzene and has sufficient electron density that can polarise bromine molecules
  • this happened because the electrons have similar energy levels so can move easily (carbon and oxygen)
32
Q

activation of phenol and phenyl benzene ring

A
  • a lone pair donates electrons to delocalised ring electron system
  • this increases the electron density making it easier to attract electrophiles
  • this means most substitutions occur at the position 2 and 4 (6 isomer of 2)
  • substitution happens faster at these carbons
33
Q

directing effects

A

how a functional group attached directly to an aromatic ring affects which carbon atoms are more likely to undergo substitution

34
Q

deactivation of nitro groups

A
  • withdraw electrons from the pi electron system

- rate of substitution is highest on the 3rd position

35
Q

2,4 directing groups (activating)

A
  • NH2 or NHR
  • OH
  • OR
  • R or C6H5
  • halogens (these aren’t activating groups)
36
Q

3 directing groups (deactivating)

A
  • RCOR
  • COOR
  • SO3H
  • CHO
  • COOH
  • CN
  • NO2
  • NR3+
37
Q

tarry products

A

formed because nitro acid is an oxidising agent and phenol is very easily oxidised

A Level Chemistry (21 decks)

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